A JWST survey of the Trapezium Cluster & inner Orion Nebula. I. Observations & overview

Abstract: We present a near-IR survey of the Trapezium Cluster and inner Orion Nebula using the NASA/ESA/CSA James Webb Space Telescope. The survey with the NIRCam instrument covers 10.9 x 7.5 arcminutes (~1.25 x 0.85 pc) in twelve wide-, medium-, and narrow-band filters from 1-5 microns and is diffraction-limited at all wavelengths, providing a maximum spatial resolution of 0.063 arcsec at 2 microns, corresponding to ~25 au at Orion. The suite of filters chosen was designed to address a number of scientific questions including the form of the extreme low-mass end of the IMF into the planetary-mass range to 1 Jupiter mass and below; the nature of ionised and non-ionised circumstellar disks and associated proplyds in the near-IR with a similar resolution to prior HST studies; to examine the large fragmented outflow from the embedded BN-KL region at very high resolution and fidelity; and to search for new jets and outflows from young stars in the Trapezium Cluster and the Orion Molecular Cloud 1 behind. In this paper, we present a description of the design of the observational programme, explaining the rationale for the filter set chosen and the telescope and detector modes used to make the survey; the reduction of the data using the JWST pipeline and other tools; the creation of large colour mosaics covering the region; and an overview of the discoveries made in the colour images and in the individual filter mosaics. Highlights include the discovery of large numbers of free-floating planetary-mass candidates as low as 0.6 Jupiter masses, a significant fraction of which are in wide binaries; new emission phenomena associated with the explosive outflow from the BN-KL region; and a mysterious "dark absorber" associated with a number of disparate features in the region, but which is seen exclusively in the F115W filter. Further papers will examine those discoveries and others in more detail.

• The paper identifies numerous free-floating planetary-mass objects, including candidates below one Jupiter mass, challenging current star formation theories.
• The study characterizes circumstellar disks in silhouette using multi-filter NIRCam imaging to reveal detailed disk properties in a high UV environment.
• The survey uncovers complex outflows and jet interactions, particularly in the BN-KL region, offering new insights into dynamic stellar feedback processes.

Overview of the JWST Survey of the Trapezium Cluster in the Orion Nebula

This paper presents an observational survey of the Trapezium Cluster and the inner Orion Nebula using the James Webb Space Telescope (JWST). Conducted with the NIRCam instrument, the survey covers an area of 10.9 × 7.5 arcminutes, equivalent to approximately 1.25 × 0.85 parsecs. This study offers significant insights into star formation by observing the region with twelve filters ranging from wide to narrow-band across wavelengths from 1 to 5 microns, achieving a diffraction-limited spatial resolution of 0.063 arcseconds at 2 microns.

Key Scientific Objectives

The primary scientific aims of the survey are centered around three main objectives:

1. Investigation of Low-mass Objects: The study aims to probe the extreme lower end of the initial mass function by identifying free-floating planetary-mass objects (PMOs) with masses below 1 Jupiter mass. Existing models suggest these objects possess temperatures conducive to distinct absorption features from water and methane, which have guided the selection of specific medium-band filters for their identification.
2. Characterization of Circumstellar Disks: The survey seeks to measure the sizes of circumstellar disks, especially those visible in silhouette against the nebula. Comparisons between near-infrared and visible observations allow for the characterization of disk properties and the investigation of the interaction between these disks and the bright UV environment of the Trapezium OB stars.
3. Study of Outflows and Jets: The survey observes explosive outflows from the BN-KL region and searches for new jets emerging from young stars in the Trapezium Cluster. Specific narrow-band filters targeting molecular hydrogen and [Fe II] emission lines have been utilized to explore these features in detail.

Methodology

The methodology involves using the NIRCam's multi-channel capabilities to; simultaneously capture SW (short wavelength) and LW (long wavelength) observations. Multiple dither positions and a mosaic strategy ensure comprehensive spatial coverage and high-quality imaging. Data reduction was performed using a customized JWST pipeline to create large-scale color mosaics. This process involved correcting for astrometric errors and applying advanced cleaning techniques to remove artefacts such as persistence and cosmic rays.

Key Findings

Several exciting discoveries have emerged from this survey:

• Free-floating Planetary-Mass Candidates: The study identifies numerous free-floating planetary-mass candidates, including some objects with masses as low as 0.6 Jupiter masses. Interestingly, a significant proportion of these are found in wide binary systems (JuMBOs), potentially challenging current theories on their formation.
• Morphological Features of the Nebula and Disks: The survey reveals intricate details of the Bright Bar and jet structures within the nebula. The high spatial resolution enables observations of ionized proplyds and the dynamics of circumstellar disks influenced by UV radiation from nearby massive stars.
• Outflows and Jets: New manifestations within the massive outflow from the BN-KL region and other jets from young stellar objects have been identified. The survey exhibits structures indicative of complex interactions with the surrounding medium.
• Mysterious Absorption Features: A peculiar "dark absorber" phenomenon, visible exclusively in the F115W filter, suggests intriguing absorption dynamics possibly involving helium or silicon line interactions, warranting further investigation.

Implications and Future Directions

The findings have robust implications for understanding star and planet formation, particularly within dense cluster environments. The identification of low-mass objects and their binary nature could inform and possibly revise existing formation and evolutionary models. Furthermore, by exposing previously unseen features and behaviors in stellar environments, the results challenge the completeness of current theoretical models and emphasize the complexity of stellar birthplaces.

This survey also underscores the potential of JWST to transform our perceptions of classic astronomical laboratories such as the Orion Nebula. Future research can build upon these results through additional JWST spectroscopic campaigns and continued analysis of the dynamic interactions taking place within these young stellar regions. Upcoming observations in Cycle 2 of the JWST mission are planned to provide further spectral insights into the detected PMOs, aiding in the confirmation and precise characterization of their planetary natures.